Macular pigment is the yellow pigment situated in the central portion of the human retina. The absorption spectrum for the pigment has a peak for light of a wavelength of 460 nm and is zero for light of a wavelength of 540 nm, so that the pigment absorbs significant amounts of the shorter wavelength light, whilst having very little or no effect on the light of the longer wavelength.
The highest concentrations of macular pigment are found in the region of the retina which has a very high number density of cone receptors, and is coupled to a disproportionately large area of the visual cortex, giving that region a high degree of visual acuity.
It has been proposed that the macular pigment protects the retina against harmful effects of short wavelength radiation, and accordingly much effort has been devoted to measuring the optical density of macular pigment in various subjects in order to determine whether there is any correlation between irregularities in the amount of macular pigment present in various subjects and certain sight defects.
To that end, a flicker photometer projects green and blue light (respectively of wavelengths of 540 nm and 460 nm) into a subject's eye in an alternating sequence. The subject is able, using a graded filter, to vary the intensity of light of one of those wavelengths. If the incidence of the different wavelengths of light on the subject's eye alternates at an appropriate frequency, the subject will be able to select an intensity of one of the wavelengths which is such that the subject perceives no, or a minimum amount of, flickering.
When this condition is achieved, the position of the filter, and hence the intensity of the light of one of the colours, is calculated, and that calculation is used to provide an indication of the optical density of macular pigment.
One example of such a photometer is described in the article "Comparison of Macular Pigment Densities in Human Eyes (R A Bone and J M B Sparrock: Vision Res. Vol. 11, pp 1057-1064. Pergamon Press 1971). This photometer has two separate light sources which generate two beams which are passed through appropriate colour filters. Each beam is directed on to a respective screen which scatters light from that beam towards the eye of the subject. One of the screens forms part of a sectored disc which is, in use, rotated by a motor and extends into the path of light from the other screen to the eye. The sectored disc thus acts as a shutter which causes the light incident on the eye from one beam to alternate with the light incident on the eye from the other beam.
A disadvantage of this type of photometer is that it is a relatively large piece of equipment which is not readily portable.